A challenge in the production of ultrafiltration membranes is to create a membrane having pores which are small enough to allow effective retention of macromolecules and at the same time maintaining an acceptable flow rate of the fluid to be filtered. This goal becomes increasingly difficult to achieve with decreasing molecular weight cut-off, i.e., decreasing pore size in the separation layer of the membrane, as there normally is a correlation between pore size and hydraulic permeability. Several proposals have been made to overcome this limitation. Examples include providing the membrane surface with permanent charges to increase rejection, or combining the fine pored separation layer with a microporous support structure, respectively.
US 2006/096912 A1 discloses cellulose hydrate ultrafiltration membranes having improved filtration performance, in particular an improved ratio of the membrane's flux to its rejection, and a process for their production. The process comprises tempering a cellulose ester membrane and saponifying the tempered membrane.
US 2003/178368 A1 discloses charged cellulosic filtration membranes produced by covalently modifying the membrane's surface with a charged compound or a compound capable of being chemically modified to possess a charge.
U.S. Pat. No. 4,824,568 A discloses composite ultrafiltration membranes based on PVDF which are produced by forming an ultrafiltration membrane structure on a preformed microporous membrane. A polymeric coating is cast onto a microporous membrane's surface and then crosslinked in place to provide a charge to the membrane.
WO 96/02317 A1 discloses a cellulosic ultrafiltration membrane made from a microporous base resistant substrate and a thin cellulose or cellulose ester polymer ultrafiltration layer.
EP 1 842 581 B1 discloses a method for preparing highly asymmetric ultrafiltration membranes. The flat sheet ultrafiltration membranes are prepared by casting a film of a solution or dispersion of a sulfone polymer and coagulating the film in a quench bath. The membranes show improved flow rates and have a high degree of asymmetry between the size of pores at the skin face of the membrane and the size of pores at the opposite face of the membrane.
WO 2004/056459 A1 discloses permselective asymmetric hollow fiber membranes comprising at least one hydrophobic polymer and at least one hydrophilic polymer. The membranes have an outer surface having 10,000 to 150,000 pores per mm2 with a pore size in the range of 0.5-3 μm. Low flux membranes having a selective layer with pores sized 5-10 nm and a cut-off of about 5 kDa in the presence of whole blood are disclosed. In aqueous solutions, cut-off will be much larger than 5 kDa.
EP 2 042 230 A1 discloses hydrophilic membranes for haemodialysis and haemodiafiltration which have been treated with a nonionic surfactant. Asymmetric membranes comprising polysulfone, polyethersulfone or polyarylethersulfone, and polyvinylpyrrolidone are disclosed which have an average pore size below 6 nm, and a sieving coefficient for albumin in whole blood of below 0.01. The ultrafiltration membranes are prepared using a center fluid containing 40-50 wt % NMP and 50-60 wt % water.
It has now been found that improved porous hollow fiber ultrafiltration membranes comprising polysulfone, polyethersulfone or polyarylethersulfone, and polyvinylpyrrolidone can be obtained by a particular selection of certain parameters of the production process.